a) Why does one get “the munchies” after one smokes marijuana?
b) Why does Del Taco taste like the best food ever when one has said munchies?
In answer to (a):
According to this New Scientist article, cannabinoid chemicals in the brain are a trigger for appetite. These occur naturally, but when added to in the form of marijuana, you get the munchies.
Question one may be better answered at www.hightimes.com
As for question two, you’d have to be drunk or stoned to think Del Taco is edible. 
why do some people NOT want to eat when they smoke weed? is it just fear of overdoing it?